The radiofrequency communication systems generally include receiving and transmitting devices such as antennas, a processing unit as well as an interface enabling signals to be exchanged between the transceiver devices and the processing unit.
On one hand, to make processable by a processing unit a signal received by the receiving/transmitting device, and on the other hand, to allow to transmit a signal produced by the processing unit, the receiving and transmitting functions generally comprise amplification, filtering, mixing and modulation/demodulation steps. The mixers are, among other things, used to translate high frequency signals to lower frequency bands in order to make the processing easier. In particular, the use of some filters, calculators or demodulators sometimes requires operating at less high frequencies, called intermediate frequencies.
One of the problems encountered with the communication systems is the simultaneous transmission/reception, or at least the transmission/reception in half-duplex mode. In particular, a further difficulty arises with using mixers. Indeed, there occurs undesirable frequency signals produced upon mixing, whether in receiving phase or in transmitting phase.
In order to address these problems, it is known to use a signal transmitting string separated from a receiving string, as shown in FIG. 1. Each of these processing strings then includes its own components and can operate without disturbing the attendant string significantly. Such a structure has in particular the drawback to raise the cost and the size of the circuit. In particular, at least two mixers 5, 7 are required, one for the receiving phase that converts the high frequency signals to an intermediate frequency and the other for the transmitting phase that converts the signals from the processing unit in a higher frequency. In a millimetric frequency band, these elements can in particular be integrated on a microwave integrated circuit further called MMIC according to the Anglo-Saxon acronym “Monolithic Microwave Integrated Circuit”. However, the number and the size of the components to be integrated on this type of circuits is a crucial criterion to be taken into account in the designing phase.
FIG. 1 presents a radiofrequency transceiver system according to the prior art. The system 1 includes an antenna 2, a switching device 3, a processing unit 4, a local isolator 5 and two mixers 6 and 7.
In receiving phase, a signal SRF of frequency RF picked up by the antenna 2 is transmitted by the switching device 3 towards a first input 6a of the first mixer 6. By combining a signal SRF with a signal of frequency FOL provided on a second input 6b by the local oscillator 5, the first mixer 6 produces a signal SFI on an output 6c at an intermediate frequency compatible with the operation of the processing unit 4.
In transmitting phase, the processing unit 4 provides on a first input 7c of the second mixer 7 a signal SFI of frequency FI. By combining the signal SFI with a signal of frequency FOL provided by the local oscillator 5 on the second input 7b, the second mixer 7 produces on an output 7c a signal SRF′ of frequency RF. The signal SRF′ is then transmitted by the switching device 3 to the antenna 2 that can transmit it.